Conventionally in the substrate with circuit design such as printed circuit boards, flex printed circuit boards, and packaging substrate, contact pads for electrical connection are formed on the surface of these substrates such as aluminum pads or copper pads. By means of bumping, solder ball placement or solder paste printing, contact pads are used for bonding an exterior electrical component, such as flip chip, printed circuit board, or passive component. There are two kinds of contact pads on the substrate, SMD (Solder Mask Defined) and NSMD (Non-Solder Mask Defined), where the dimension of the SMD contact pads are larger than the openings of the solder mask and the dimension of the NSMD contact pads are smaller than the openings of the solder mask, moreover, the edge of the NSMD contact pads will not be covered by the solder mask. For SMD contact pads, the exposed area of the contact pads is defined by the solder mask to improve the fixing ability of contact pads on the substrate. However, since the edge of the opening of the solder mask will contact with bumps, solder balls or solder paste, therefore, bumps, solder balls or solder paste will experience extra stress from the solder mask which will cause poor adhesion or even failure which is revealed in Taiwan Patents No. 441059. For NSMD contact pads, the upper surfaces and the sidewalls of the contact pads are free from solder mask which can improve the wettability of solder balls or solder paste.
As shown in FIGS. 1 and 2, a conventional substrate 10 is disclosed that has a plurality of NSMD contact pads 20 thereon. Each contact pad 20 has a circular bonding surface 21 and a sidewall 22. Each contact pad 20 is connected with a trace 30 of metal wiring layers on same surface. The solder mask 40 is applied on the surface of the substrate 10 and has a plurality of openings 41. The openings 41 are larger than the contact pads 20 and expose the bonding surfaces 21 and the corresponding sidewalls 22 of the contact pads 20 for bumping, solder balls placement or solder paste printing. However, as shown in FIG. 1 the connecting ends 31 of the traces 30 which are connected to the NSMD contact pads 20 are also exposed out of the openings 41 of the solder mask 40. Any external stress or force experienced on the contact pads 20 also directly impact the exposed connecting ends 31 of the traces 30. This will easily cause the traces 30 to break or electrically fail. Moreover, because contact pads 20 are completely exposed out of the solder mask 40, therefore, the positions of contact pads 20 on the substrate 10 may be shifted which will cause alignment problems in the following assembly processes and the assembly yield will be low.